Precise knowledge of the mechanical properties of rock formations during wellbore construction is of utmost importance, especially in the hydrocarbon exploration and production industry. Knowledge of formation properties may be used to improve drilling operations, to design well completions, and/or to enhance hydrocarbon production during hydro-fracturing.
Estimating a rock formation's mechanical properties may be accomplished prior to drilling, after drilling, or during drilling. One approach for sensing formation properties uses seismic data acquired away from the drill bit (e.g., at the surface of a borehole). Because this approach senses at a distance, the estimates of the formation properties may suffer from low spatial resolution and/or error. Another approach (e.g., wireline formation testing) may sense formation properties in the borehole (i.e., wellbore) after the wellbore has been drilled. This approach may offer improved accuracy because it senses in the wellbore but suffers because the stresses, pore fluids, and pressures may have been altered by the drilling process. For example, measuring formation properties in the wellbore while drilling may be accomplished using logging while drilling (LWD) or measuring while drilling (MWD) tools that are integrated with a drill string. Because the LWD/MWD tools are typically integrated with the drill string away from the drill-bit (e.g., 10-60 feet behind), the formation properties measured have already been changed as a result of the drilling. Further, LWD/MWD tools may have associated safety/regulatory issues because they generally use radioactive sources to probe a formation. Recent research has shown that it is possible to estimate formation properties at the drill-bit by sensing acoustic noise stemming from rock failure during drilling. This approach relies on drilling noise (e.g., normal vibrations associated with drilling) to excite desired mechanical harmonics from which the formation properties may be inferred. This approach is limited, however, because the drilling noise relied on for sensing and measurement is uncontrolled. For at least this reason, this approach has not been generally used for commercial drilling.
A need, therefore, exists for an apparatus, system, and method for obtaining high-resolution and reliable estimates of formation properties (i) at the drill-bit, (ii) during wellbore construction, and (iii) with improved control over the sensing and measurement.